combinations of direct, low-frequency, audio-frequency, and radio-frequency currents may exist. It is frequently necessary to have a means for separating these component currents at any desired point. An electrical device for accomplishing this separation is called a FILTER. ">

In many practical applications of complex circuits, various combinations of direct,
low-frequency, audio-frequency, and radio-frequency currents may exist. It is frequently
necessary to have a means for separating these component currents at any desired point. An
electrical device for accomplishing this separation is called a FILTER.

A filter circuit consists of inductance, capacitance, and resistance used singularly or
in combination, depending upon the purpose. It may be designed so that it will separate
alternating current from direct current, or so that it will separate alternating current
of one frequency (or a band of frequencies) from other alternating currents of different
frequencies.

The use of resistance by itself in filter circuits does not provide any filtering
action, because it opposes the flow of any current regardless of its frequency. What it
does, when connected in series or parallel with an inductor or capacitor, is to decrease
the "sharpness," or selectivity, of the filter. Hence, in some particular
application, resistance might be used in conjunction with inductance or capacitance to
provide filtering action over a wider band of frequencies.

Filter circuits may be divided into four general types: LOW-PASS, HIGH-PASS, BANDPASS,
AND BAND-REJECT filters.

Electronic circuits often have currents of different frequencies. The reason is that a
source produces current with the same frequency as the applied voltage. As an example, the
a.c. signal input to an audio amplifier can have high- and low-audio frequencies; the
input to an rf amplifier can have a wide range of radio frequencies.

In such applications where the current has different frequency components, it is
usually necessary for the filter either to accept or reject one frequency or a group of
frequencies. The electronic filter that can pass on the higher-frequency components to a
load or to the next circuit is known as a HIGH-PASS filter. A LOW-PASS filter can be used
to pass on lower-frequency components.

Before discussing filters further, we will review and apply some basic principles of
the frequency-response characteristics of the capacitor and the inductor. Recall the basic
formula for capacitive reactance and inductive reactance:

Assume any given value of L and C. If we increase the applied frequency, XC
decreases and XL increases. If we increase the frequency enough, the capacitor
acts as a short and the inductor acts as an open. Of course, the opposite is also true.
Decreasing frequency causes XC to increase and XL to decrease. Here
again, if we make a large enough change, XC acts as an open and XL
acts as a short. Figure 1-13 gives a pictorial representation of these two basic
components and how they respond to low and high frequencies.

If we apply these same principles to simple circuits, such as the ones in figure 1-14,
they affect input signals as shown. For example, in view (A) of the figure, a low
frequency is blocked by the capacitor which acts as an open and at a high frequency the
capacitor acts as a short. By studying the figure, it is easy to see how the various
components will react in different configurations with a change in frequency.

Figure 1-14. - Reaction to circuit by change in frequency.

As mentioned before, high-pass and low-pass filters pass the specific frequencies for
which circuits are designed.

There can be a great deal of confusion when talking about high-pass, low-pass,
discrimination, attenuation, and frequency cutoff, unless the terms are clearly
understood. Since these terms are used widely throughout electronics texts and References,
you should have a clear understanding before proceeding further.

HIGH-PASS FILTER. A high-pass filter passes on a majority of the high frequencies to
the next circuit and rejects or attenuates the lower frequencies. Sometimes it is called a
low-frequency discriminator or low-frequency attenuator.

LOW-PASS FILTER. A low-pass filter passes on a majority of the low frequencies to the
next circuit and rejects or attenuates the higher frequencies. Sometimes it is called a
high-frequency discriminator or high-frequency attenuator.

DISCRIMINATION. The ability of the filter circuit to distinguish between high and low
frequencies and to eliminate or reject the unwanted frequencies.

ATTENUATION. The ability of the filter circuit to reduce the amplitude of the unwanted
frequencies below the level of the desired output frequency.

FREQUENCY CUTOFF (fco). The frequency at which the filter circuit changes
from the point of rejecting the unwanted frequencies to the point of passing the desired
frequency; OR the point at which the filter circuit changes from the point of passing the
desired frequency to the point of rejecting the undesired frequencies.

LOW-PASS FILTER

A low-pass filter passes all currents having a frequency below a specified
frequency, while opposing all currents having a frequency above this specified frequency.
This action is illustrated in its ideal form in view (A) of figure 1-15. At frequency
cutoff, known as fco the current decreases from maximum to zero. At all
frequencies above fco the filter presents infinite opposition and there is no
current. However, this sharp division between no opposition and full opposition is
impossible to attain. A more practical graph of the current is shown in view (B), where
the filter gradually builds up opposition as the cutoff frequency (f co) is
approached. Notice that the filter cannot completely block current above the cutoff
frequency.

Figure 1-15A. - Low-pass filter.

Figure 1-15B. - Low-pass filter.

View (A) of figure 1-16 shows the electrical construction of a low-pass filter with an
inductor inserted in series with one side of a line carrying both low and high
frequencies. The opposition offered by the reactance will be small at the lower
frequencies and great at the higher frequencies. In order to divert the undesired high
frequencies back to the source, a capacitor must be added across the line to bypass the
higher frequencies around the load, as shown in view (B).

Figure 1-16A. - Components of a simple low-pass filter.

Figure 1-16B. - Components of a simple low-pass filter.

Figure 1-16C. - Components of a simple low-pass filter.

The capacitance of the capacitor must be such that its reactance will offer little
opposition to frequencies above a definite value, and great opposition to frequencies
below this value. By combining the series inductance and bypass capacitance, as shown in
view (C), the simplest type of low-pass filter is obtained. At point P, a much higher
opposition is offered to the low frequencies by the capacitor than by the inductor, and
most of the low-frequency current takes the path of least opposition. On the other hand,
the least amount of opposition is offered to the high frequencies by the capacitor, and
most of the high-frequency energy returns to the source through the capacitor.